Line commutated converters (LCC) are a type of current source converter (CSC) and can be employed in high voltage direct current (HVDC) transmission links between high voltage alternating current (HVAC) power networks or grids. LCCs employ naturally commutated thyristor bridges in rectifying (power transmitting) and inverting (power receiving) terminals. Power flow through such HVDC transmission links may be reversible whilst HVDC transmission current is unipolar. HVDC transmission link voltage must also be reversible and this can be achieved using the well known principle of phase control. The requirement for a reversible HVDC transmission link voltage imposes severe limitations upon the choice of HVDC cable insulation and this renders the use of near ideal elastomeric cable insulation systems impossible. The requirement for a reversible HVDC transmission link voltage also imposes severe limitations upon the potential adoption of multi-terminal HVDC grid networks, which are only fully effective in permitting reversible power flow at all terminals when a unipolar HVDC transmission link voltage is employed.
Naturally commutated thyristor bridges are the most efficient and reliable means of AC to DC and DC to AC power conversion, but this advantage is to a significant degree opposed by a requirement to correct the HVAC grid terminal power factor and harmonic distortion that is caused by the use of the principle of phase control. Phase control is used to regulate the power flow through the HVDC transmission link whilst respective HVAC grid voltages vary and, historically, very large switched passive filters and power factor correction networks have been used to compensate for the HVAC grid terminal power factor and harmonic distortion that is caused by the use of phase control. More recently, static compensators have been employed to simplify and reduce the need for switched compensation systems. The requirement to compensate harmonic distortion and power factor is exacerbated as the ranges of respective HVAC grid voltages and associated ranges of phase control are increased. Safe inverting LCC commutation cannot be achieved during severe HVAC grid voltage dips since the required range of phase control is inconsistent with the requirements for thyristor commutation and, for the same reason, inverting LCCs cannot energise an HVAC grid system that has become de-energised. Despite the above disadvantages, the power transmission efficiency of LCCs is such that they are typically the preferred solution for high power point-to-point HVDC transmission links.
Standard Number PD IEC/TR 62544-2011 (High-voltage directly current (HVDC) systems—application of active filters) anticipates DC side harmonic mitigation using series and shunt mode and hybridised DC side active filters; AC side harmonic mitigation using shunt and series mode and hybridised AC side active filters; and DC link coupling between shunt and series mode components of a hybrid active filter system. These active systems employ force commutated power electronic circuits which add complexity to the otherwise inherently simple and robust LCC topology.
CSCs employing force commutated thyristors have been proposed as a means of mitigating some of the above deficiencies of LCCs, but the proposed circuits include large and complex auxiliary commutation circuits in addition to the otherwise inherently simple and robust LCC topology.
More recently, voltage source converters (VSCs) have been increasingly used in HVDC transmission link systems having moderately high power ratings. VSCs achieve reversible power flow by allowing HVDC transmission link current to reverse, thereby allowing the use of a unipolar HVDC transmission link voltage and cables with near ideal elastomeric insulation systems. VSCs also address the HVAC grid harmonic and power factor limitations of LCCs. Moreover, VSCs, in having near independent control of real and reactive power, have been able to assist HVAC grid frequency and voltage stability. All converters are subject to compromise. Whilst VSCs overcome known limitations of LCCs, they incur the penalty of increased power losses or reduced efficiency. In most practical applications, VSCs have not been able to limit HVDC transmission link short circuit (or low resistance) fault current. As a result, recent development activity in VSCs has been directed to try and overcome these penalties.
VSC technologies have developed in the following four evolutionary stages:
Two-level pulse width modulated (PWM) VSCs with series-connected IGBTs first addressed the limitations of LCCs but the efficiency penalty was serious and DC fault current could not be limited. Large high performance passive filters were used to minimise the effects of the PWM arm voltages. HVDC transmission link short circuit fault current included a substantial inrush component from the DC link capacitor.
Three-level PWM VSCs with series-connected IGBTs improved the trade off between efficiency, passive filter specification, and VSC benefits. HVDC transmission link short circuit fault current included a reduced but still substantial inrush component from the DC link capacitor.
Multi-level modular converter (MMC) VSCs have recently been derived from existing two-level VSC elements by series connecting chains of modules which each contain basic VSC elements. These chains of modules are connected in a bridge topology and stepwise approximations of sinusoidal and anti-phase sinusoidal voltages are synthesized respectively in the positive and negative arms of each particular phase. The most primitive half-bridge MMC has greater efficiency than the three-level PWM VSC, but is unable to limit DC fault current whereas the H-bridge MMC is able to limit DC fault current but has nearly twice the power loss of the half-bridge MMC. Each MMC module has a DC link capacitor whose voltage must be controlled by regulating MMC module power flow and whose capacitance is sufficient to limit module DC link voltage ripple. These MMCs employ extremely complex IGBT firing sequences in order to synthesise stepwise voltage waveforms which must adapt to HVAC grid voltage and line current whilst also adapting to the effects of component failures within MMC modules and regulating DC link capacitor voltage. It is because individual IGBTs switch at HVAC grid fundamental frequency and switching losses are minimal that MMC VSC efficiency is far greater than that of PWM VSCs. The stepwise synthesis of arm voltages has allowed the size and complexity of passive filters to be reduced relative to that in PWM VSCs. The MMC VSC has revolutionised the scope of application of VSCs by being modular and scaleable to high power and high voltage ratings. HVDC transmission link short circuit fault current still includes a substantial inrush component from the DC link capacitors, but this may be partially mitigated by employing a protective firing sequence.
Most recently the hybridisation of conventional quasi-square wave VSC bridges comprising series-connected IGBTs with MMC VSCs that are configured to act as DC link shunt mode or in-arm series mode active filters has been disclosed as a means of combining the harmonic and power factor mitigating capabilities of the MMC VSC with the efficient power handling of the quasi-square wave VSC. Zero voltage switching is achieved in the IGBTs in the quasi-square wave VSC circuits. The hybrid arrangement with DC shunt mode MMC VSC active filtration cannot limit DC fault current, but benefits from a MMC of reduced complexity relative to three-phase MMC VSC and has a greater efficiency than three-phase MMC VSCs. The ability of the shunt mode MMC VSC active filter to mitigate the effects of DC and AC side harmonic effects simultaneously is subject to significant compromise. The hybrid arrangement with in-arm series mode MMC VSC active filtration can limit DC fault current, but suffers the penalty of having a MMC whose complexity is comparable to that of a three-phase MMC VSC and has a lower efficiency than that of the DC shunt mode MMC filtered hybrid. As a result of employing VSC technology throughout these hybrid circuits, HVDC transmission link short circuit fault current still includes a substantial inrush component from the MMC module DC link capacitors, but this may be partially mitigated by employing a protective firing sequence.
Accordingly, the present invention seeks to better hybridise an efficient rectifying and inverting power conversion circuit that can rapidly limit AC and DC side fault current, with active DC ripple, AC harmonic and power factor mitigating circuits.